Welcome to the forum. This is definitely the place to get answers!
The quality of the milk definitely makes a difference with any cheese recipe ... except, maybe, ricotta, which is what you wound up making in the end. And yes, the lack of cultures will certainly be bland ... sort of just the taste of milk with a bit of lemon in it.
If you didn't add salt, that will certainly help.
The two things that damage store-bought milk are heat and homogenization. You can
successfully make cheese with pasteurized and homogenized (P&H) milk, but not
pasteurized (UP) milk. If the milk you used was UP, that would explain the problem. Even with just pasteurized (not UP) milk, though, results can vary from brand to brand - some brands of milk seem to be pasteurized at higher temps than others. So the key to success in using store-bought milk is to try various brands until you find one that works. If you can get low-temperature pasteurized (LTP), cream-line (non-homogenized) milk, that will give very, very good results. Of course, the very best results will come from fresh raw milk.
One other thought - I have never made Queso, so I don't know what normally goes into that recipe. But I am wondering about the temperature -- if I recall correctly, rennet begins to work less well as you get up over 100°, and at 130° the enzymes from the rennet are destroyed. Don't take that for gospel, since it is a fuzzy memory on my part - hopefully someone else will correct me on this if I'm not remembering correctly!